/Engine/Source/Runtime/Core/Public/GenericPlatform/GenericPlatformMisc.h
Converts from a local user index to aFPlatformUserIdA platform-allocated opaque user id, or PLATFORMUSERID_NONE
static [FPlatformUserId](API\Runtime\Core\Misc\FPlatformUserId) GetPlatformUserForUserIndex ( int32 LocalUserIndex )